Implants adapted for use in the lumbar spine and the thoracic spine become much less usable in the cervical spine because of differences in anatomy. In the lumbar spine, the disc spaces are about 25% as tall as the vertebral bodies (i.e., the vertebral bodies are generally four times taller than the intervening disc space). In the cervical spine, the disc space can be 50% of the height of the vertebral bodies. The disc spaces in the cervical spine are generally not greater than 7 or 8 mm tall in most people.
Screws generally used to secure an implant in the cervical spine typically have a diameter of between 4 and 5 mm. If two bone screws were to be inserted one each into each of the adjacent cervical vertebral bodies; and if one were to attempt to vertically oppose those two bone screws, this would not prove possible because the sum of the screw diameters would exceed the height of the implant. Such vertically aligned bone screws would require at least 10 mm of combined height for themselves plus sufficient implant structure and further height sufficient to surround and retain them. Thus, altogether the two bone screws and the surrounding implant would have to have a combined height that would substantially exceed the height of the disc space and an implant adapted to fit therein.
Alternatively, one could try to place a number of bone screws more horizontally (side-by-side) so as to avoid the problems described above associated with vertical alignment. To provide for the preferred implant stability that the use of paired screws would provide (two each into each of the adjacent vertebral bodies), one could horizontally align four bone screws on the equator of the implant with two of the bone screws directed toward one of the cervical vertebral bodies and two of the bone screws directed toward the other of the adjacent cervical vertebral bodies. Four such horizontally aligned bone screws having a head diameter of 5 mm each would require at least 20 mm for the screw heads alone. Further, with sufficient implant structure to surround each of those screw heads, the implant width would at a minimum be about 24 mm, which would exceed the desirable implant width for most cervical disc spaces. Staggering the bone screw receiving holes would be of some benefit, but of itself not an adequate solution to the problem described where it is desirable to maintain some symmetry of the screws to each other, the vertebrae, and the implant.
One prior art solution to the aforementioned problem teaches extending the height of the trailing end of the implant to make it taller than the disc space. An example of this is a flanged implant. The flanged implant makes it possible to place screws so that they can be vertically aligned and have sufficient structure of the implant to retain them. The flanged portion of the implant, however, extends outside of the disc space which may not be desirable in all circumstances. Further, these flanged implants may not be usable when it is needed to fuse multiple levels of the spine.
Accordingly, there exists a need for a spinal implant adapted to provide the advantages of a flanged implant for placement and orientation of bone screws associated therewith but without the flanged portion, or the necessity of the implant extending outside of the disc space.